The present invention relates to a deposition technique for metal oxide films and more particularly to the polymer assisted aqueous solution deposition of metal oxide films, especially thin metal oxide films. The present invention also relates to the preparation of metal nitride films.
Metal oxide ceramic thin films are widely used in the electronics industry. Preparation of such metal oxide ceramic films has been accomplished by physical vapor deposition techniques, chemical vapor deposition techniques, and by chemical solution deposition techniques. Chemical solution deposition techniques have been generally viewed as less capital intensive (see, Lange, xe2x80x9cChemical Solution Routes to Single-Crystal Thin Filmsxe2x80x9d, Science, vol. 273, pp. 903-909, 1996 and Schwartz, xe2x80x9cChemical Solution Deposition of Perovskite Thin Filmsxe2x80x9d, Chem. Mater., vol. 9, pp. 2325-2340, 1997). Yet, typical chemical solution deposition techniques are carried out in an organic solvent. Organic solvents are not desirable for many industrial production lines. For example, well known sol-gel processes typically employ a metal alkoxide together with an organic solvent such as 2-methoxyethanol. The elimination of organic solvents from chemical solution deposition techniques is generally preferred when possible. Thus, continued efforts have been expended in the development of chemical solution deposition processes which can be organic solvent-free.
Another drawback of using organic solvents is their compatibility with organometallic compounds. Metal salts are typically insoluble in organic solvents. In order to make metal salts or compounds soluble in organic solvents, organic moieties have to be added to form metal complexes. This is often undesirable as new reactions and techniques must be developed to incorporate such soluble organic groups onto metal ions. The resulting organometallic compounds are usually difficult to handle because of their relatively higher reactivity than metal salts.
One common chemical solution method, a sol-gel process, uses the high reactivity of organometallic precursors and hydrolyzes these organometallic compounds to make various oligimers. These metal oxo oligimers have suitable viscosity to allow spinning into thin films, which can be fired into ceramic materials at high temperatures. The complication in such a sol-gel process is the uncontrollable polymerization of the metal oxo oligimers because of complex reactive species in the precursor solution. Therefore, the reproducibility of sol-gel processes is poor which hinders the applications in industrial processes despite the low costs.
In the processing of metal salts into metal oxides, the use of organometallic compounds as well as sol-gel techniques can be less desirable than an aqueous process. However, one problem with aqueous processing of metal salts into metal oxide thin films is the hydrolytic properties of the metal ions. Typically, metal salts such as metal nitrates or metal acetates are very soluble in water and thee metal ions undergo various degrees of hydrolysis. For example, transition metal ions, such as titanium, niobium and tantalum, react with water violently to yield metal hydroxides and a smoke-like hydrolytic side product of hydrogen chloride. This is undesirable because these transition metals may precipitate out of solution prior to further processing. Therefore, the problems of metal ion hydrolysis must be solved to allow an aqueous solution method for processing metal oxides. A more desired methodology to achieve ceramic thin films would be a chemical solution having the following properties: clean decomposition to pure ceramics; stable chemical solutions (no gelling) without any reactions before the firing stage; and, the desired viscosity for spin coatings or film casting.
U.S. Pat. No. 5,368,834 describes a method for producing a metal titanate powder such as barium strontium titanate powder by forming a solution of metal organic precursor compounds including, e.g., a titanium lactate chelate, placing the solution in a reservoir, forming a mist from the solution, entraining the droplets of the mist in a carrier stream and subjecting the droplets to a temperature of at least 700xc2x0 C. for a period of time to pyrolyze the droplets into a powder of the metal titanate. The powder can then be formed into a ceramic shape by combining the powder with an organic binder, pressing into the desired shape and sintering to form the final ceramic material. There is no description of forming a film of the metal titanate directly from the solution without initially forming a powder.
An object of the present invention is to provide a chemical solution deposition method of forming metal oxide films, such a chemical solution deposition method including the deposition of a water-soluble metal complex and a water-soluble polymer.
Another object of the present invention is to provide a chemical solution deposition method of forming high purity metal oxide thin films.
Another object of the present invention is to provide a chemical solution deposition method of forming metal nitride films, such a chemical solution deposition method including the deposition of a water-soluble metal complex and a water-soluble polymer.
Yet another object of the present invention is to provide a metal oxide or metal nitride precursor solution that has a longer shelf-life time than typical sol-gel solutions as such solution to ceramic approaches offer flexibilities and convenience frequently required in manufacturing processes.
Still another object of the present invention is to provide an aqueous chemical solution deposition method of forming metal oxide films, such a method characterized as organic solvent-free.
To achieve the foregoing and other objects, and in accordance with the purposes of the present invention, as embodied and broadly described herein, the present invention provides for a process of preparing a metal oxide film including applying an aqueous solution including a water-soluble polymer and a water-soluble metal precursor onto a substrate to form a polymer and metal-containing layer thereon, treating said substrate including said polymer and metal-containing layer to form a coherent inorganic-organic composite film, and heating said substrate at temperatures characterized as sufficient to remove said polymer and form said metal oxide film.
The present invention further provides for preparation of a barium titanate thin film by deposition of an aqueous solution including barium acetate, titanium bis(ammonium lactato)dihydride and polyvinyl alcohol, followed by the drying and calcination so as to form a polymer-free barium titanate thin film. In other embodiments, ceramic films of ruthenium oxide (RuO2), magnesium oxide (MgO), strontium titanate (SrTiO3) and yttria-barium-copper oxide (YBCO) can be prepared.
The present invention further provides a process for preparation of a metal oxide thin film, the process characterized as organic-solvent free.
The present invention further provides a composition of matter comprising an aqueous solution of a metal compound and a water-soluble polymer. The water-soluble polymer can be selected from polyvinyl alcohol, polyethylene glycol, poly(acrylic acid), poly(diallyldimethyl ammonium chloride), and polyethylenimine.
The present invention further provides a process of preparing a metal nitride film including applying an aqueous solution including a water-soluble polymer and a water-soluble metal precursor onto a substrate to form a polymer and metal-containing layer thereon, treating said substrate including said polymer and metal-containing layer to form a coherent inorganic-organic composite film, and heating said substrate at temperatures characterized as sufficient to remove said polymer and form said metal nitride film.